Novak Djokovic cast off concerns over his form and mindset by easing to a 6-1 6-2 6-4 win over Diego Schwartzman in the first round of the US Open.

Since winning Wimbledon, the world number one has had off-court matters on his mind, with marriage to long-term partner Jelena to be followed in October by the birth of their first child.

He made a decidedly sluggish start to the North American hardcourt season and arrived in New York with not many matches under his belt after losing early in Toronto and Cincinnati.

But Djokovic was quick to show Argentinian Schwartzman who was boss under the lights on Arthur Ashe Stadium, racing into a 4-0 lead.

Schwartzman clearly wanted to make the most of the occasion and urged the crowd to raise the noise level after a precision lob and forehand winner helped him get on the board with a break of the Djokovic serve.

It was merely a blip, though, and 79th-ranked Schwartzman did not win another game until he was 2-0 down in the second set, finally holding serve for the first time.

The precision on Djokovic's shots was far too good for his opponent, highlighted by a forehand winner hit round the net post to gasps and cheers.

Schwartzman is much more at home on clay than hard courts but made a better fight of things in the third as Djokovic's level dropped.

The Serbian was never in any danger, though, and served out victory after an hour and 37 minutes, the final act a rather fortunate net cord.

Djokovic, who next plays France's Paul-Henri Mathieu, said: "I'm very pleased. It's never easy to start a US Open smoothly. I love playing night sessions, it's probably the most special court in the world for night sessions."

Earlier in the day, Wimbledon hero Nick Kyrgios came close to being thrown out of the tournament for swearing.

The 19-year-old Australian, who stunned Rafael Nadal in the fourth round at the All England Club, eventually made headlines of the right sort with a 7-5 7-6 (7/4) 2-6 7-6 (7/1) victory over 21st seed Mikhail Youzhny.

Kyrgios had already been given two code violations for his bad language in the third set and a third at the start of the fourth set saw him penalised a game.

One more incident would have resulted in Kyrgios being defaulted but the teenager successfully kept his tongue in check thereafter.

Kyrgios said: "I guess it was just heat of the moment. I was frustrated the way I was playing and it was just all that sort of stuff. It was just an outburst and hopefully I will be able to control that the next time I play.

"It comes from having high expectations most of the time. I have been an emotional player most of my career. Maybe I will be able to manage it in the future. It's a work in progress. It's something that's always been there."

Kyrgios went into Wimbledon as the world's highest-ranked teenager but with little public profile.

He left the tournament as the name on everyone's lips and admits the adjustment has taken some getting used to.

"It's all happened so fast," he said. "It's sunk in a little bit, but I'm still pretty amazed how fast it all happened.

"Last year I was qualifying (in New York). I knew I had a chance to qualify, but I wasn't really expected to. Now I'm winning matches in main draw grand slams. It's pretty amazing for me even still."

Experienced Russian Youzhny made the quarter-finals here last year and represents another impressive scalp for Kyrgios, who next meets Italian Andreas Seppi.

Youzhny served to make it two sets all but Kyrgios steeled himself to break back and dominated the fourth-set tie-break.

Third seed Stan Wawrinka was given a tough time by 21-year-old Czech Jiri Vesely but managed to win in straight sets, clinching a 6-2 7-6 (8/6) 7-6 (7/3) victory.

Fifth seed Milos Raonic comes into the tournament as a real dark horse after an excellent run of form that began with victory over Kyrgios to reach the semi-finals at Wimbledon and also included an ATP title in Washington.

The Canadian allowed a third-set lead to slip away against Japan's Taro Daniel but was relatively untroubled in a 6-3 6-2 7-6 (7/1) win.

The Frenchman marked himself out as a dark horse by beating Novak Djokovic, Andy Murray, Grigor Dimitrov and Roger Federer back to back to win the Masters event in Toronto earlier this month.

He saved a set point to avoid going two sets to one down against Monaco but was little troubled thereafter.

Ireland's James McGee was unable to keep his New York fairytale going.

The Irishman achieved his dream of competing in the main draw at a Grand Slam thanks to a tremendous run through qualifying at Flushing Meadows.

McGee, ranked 194, has spent his whole career until now fighting to make a living on the lowest rungs of the professional game.

A first-round clash against Kazakh Aleksandr Nedovyesov, also ranked outside the top 100, appeared to offer the opportunity for further heroics but it was his opponent who came out on top in a 4-6 6-2 6-1 7-6 (7/3) victory.